Electric wave guide



May 23, 1950 E. c. CORK El'AL ELECTRIC WAVE GUIDE Filed April 18, 1945[N TOR Edwand/ dork, l lidhael/Bgglman-fl fold mm,

ATTORNEY Patented May 23, 1950 UNITED STATES PATENT OFFICE ELECTRIC WAVEGUIDE Application April 18, 1945, Serial No. 589,068 In Great BritainSeptember 1, 1942 Section 1, Public Law 690, August 8, 1946 Patentexpires September 1, 1962 Claims. (01. 250-3353) This invention relatesto electric wave guides for the transmission of TM or transversemagnetic type electromagnetic waves,

The invention is concerned with wave guides consisting of hollowconductors, the electromagnetic energy being transmitted through adielectric medium within the conductors.

For many purposes it may be required to employ a guide which atpositions along its length contains different dielectrics. Where such aguide is required it is found that the presence of difierent dielectricscan give rise to undesirable reflections of the electromagnetic energyat the dielectric interfaces.

It is the object of the present invention to provide an improvedconstruction of wave guide having portions filled with differentdielectrics in which reflection of energy at the boundary between thedifferent dielectrics is substantially reduced or eliminated.

According to one feature of the invention a wave guide for thetransmission of electromagnetic waves having an electric component ofthe field therewithin along the axis of the guide and hereaftergenerally referred to as waves of the E or TM type is provided havingportions filled with different dielectrics wherein the transversedimensions of the guide at the boundary between said differentdielectrics are so related to the dielectric constants of the saiddifferent dielectrics and to the frequency of the E or TM type wavesthat reflection of said waves at said boundary is substantially reducedor eliminated.

The invention is of particular use where it is desired to employ twosections of waveguide of different size, one of the sections beingfilled, for example, with a solid dielectric and the other sectionhaving an air dielectric. It is known that two guides of differenttransverse dimensions employing the same dielectric can be connectedtogether and matched to avoid reflection by joining the two sections ofguide by an intermediate section in which the transverse dimensionsthereof slowly change from the section of larger size to the section ofsmaller size. If, for example, the two sections of the guide are ofcircular form in cross-section then the two sections may be connectedtogether by means of a conical section having an angle of taper notexceeding about Where such a construction is employed and one of thesections of the guide is filled with a different dielectric from theother section of the guide, then it is possible in accordance with thepresent invention to arrange the boundary between the differentdielectrics at a suitable position along the section having the slowlychanging diameter so that reflection is substantially eliminated.

In order that the said invention may be clearly understood and readilycarried into effect, it will now be more fully described with referenceto the accompanying drawings, in which:

Figure 1 illustrates a wave guide filled with difierent dielectrics, and

Figure 2 of the drawings illustrates the invention as applied by way ofexample to a wave guide aerial employing sections of wave guide ofdifierent diameter.

As shown in Figure l, the reference numeral 3 indicates a wave guidewhich is of circular form in cross-section and is designed for thetransmission of electromagnetic waves of the so-called E01 mode. Thefirst subscript indicates the order, that is, the number of full periodvariations of radial component of the field along angular coordinateswithin the guide and the second subscript the mode; that is, the numberof half period variations of angular component of field along radialcoordinates within the guide. Thus a TM wave of the mth order and nthmode is rep resented by Emn. In the form shown in Figure 1 the guide 3has a constant cross-sectional area and one portion of the guide isfilled with a dielectric K1 which dielectric may b air and anotherportion of the guide is filled with a solid dielectric K2 such as thatknown by the trade, name Polystyrene. The boundary between the differentdielectrics is arranged in the plane S-S', which is located atright-angles to the axis of the guide. The arrangement is identical withthe system formed by joining two wave. guides of the same diameter buthaving difier-v ent dielectric materials. To insure a match between thetwo wave guides, it is necessary that the radius of the guide 3, thedielectric constants and the frequency of the waves transmitted shouldbe related so that a minimum of reflection occurs at the surface SS.

It is known that wave propagation in a guide may be regarded as a caseof reflection, there being a certain system of sources inside the guideand the total field inside the guide being the sum of the impressed andreflected fields. For example in Figure 1, the wave reaching theboundary SS is the sum of the fields of the waves refiected from thesurface of the guide enclosing the medium K1. Such reflected waves reachthe boundary SS at an angle to the normal to the SS determined by thedimensions of the guide.

This angle is important in that it determines the amounts oftransmission and reflection that occur at SS. In particular, inaccordance with this invention, when the wave is of transverse magneticor TM type and the angle is equal to the Brewster or pseudo-Brewsterangle, then reflection is wholly or substantially eliminated and thewave is effectively totally transmitted into the medium K2. This is thenequivalent to having the guide containing K1 matched to the guidecontaining K2.

The above explanation is based upon well known optical theory. A moredetailed discussion where the optical theory is identified with ageneral electromagnetic theory is to be found in SchelkunoifsElectromagnetic Waves (published in 1943) pages 251-254 while the use ofthe Brewster angle to obtain impedance matching at a surface ofdiscontinuity is referredto on page 253.

If instead of the wave guide 3 having a constant diameter as shown inFigure 1, two sections oftthe wave guide are employed having differentdiameters, then as stated above it is known that these two sections canbe matched without introducing appreciable reflection providing the twosections of different diameter are joined by an intermediate sectionhaving a slowly changing diameter. Consequently, in the case where thetwo sections of difierent diameter are filled with different dielectricsa minimum of reflection will occur at the boundary between the twodielectrics providing the boundary is located at a position where theradius of the intermediate section has a value in accordance with theabove considerations.

Figural? of the drawings illustrates the invention as applied to a waveguide aerial having two sections of different diameter joined by anintermediate section. As shown in Figure 2 the section of guideof largerdiameter is indicated bythe reference numeral 4 and the section ofsmaller diameter is indicated by the reference numeral 5 and constitutesthe aerial section of the guide. Energy can be radiated from the aerialor picked up by the aerial via a slot 6 provided on the circumference ofthe guide and, in theconstruction shown, in order that the section S'canhave a minimum diameter and at the same time the slot 6 can be protectedagainst the ingross of foreign matter, the section 5 is filled'with asolid dielectric indicated at 1, such as the material known by the tradename Polystyrene. In: order to provide a match between the sections and5 of diiierent diameter an intermediate section 8 -is' provided ofconical form having a small angle of taper. Providing the angle of taperis sufliciently small, substantially no reflection will occur at theintermediate section. In accordance with the invention the boundarybetween the solid dielectric l and the air dielectric of the section 4is so disposed that it is located at a position where the radius of thesection 8 has a value as defined by the above theory. In this manner aminimum of reflection will occur at said boundary. The end of thesection 5 is closed by a metal disc 5 to prevent the escape of energy atthe end of the section 5, and in order to prevent currents fiowing onthe outside of the section 5, the latter is eifectively earthed at aplurality of spaced points by providing two pairs of segmental fins Inof conducting material secured to bands II, in turn attached to theouter surface of the guide 5, the members Ill extending around theperiphery of the guide for about 90". These members H] are proportionedso that, in

4 effect, they have a radial length equivalent to a quarter of awavelength of the transmitted waves. The fins of each pair are alsospaced apart by a distance of approximately a quarter of a wavelength.

The invention is not limited to the above mentioned examples but may, onthe contrary, be applied to any transverse magnetic type of wave and anycrosssection of wave guide provided that the dimensions of the waveguide are such as to cause the component waves to fall on the boundaryat the Brewster angle. In other words, the dimensions of the guide mustbe such that the impedance of' the guide on one side of a transverseboundary surface is equal to the impedance on the other side of thetransverse boundary surface.

What we claim is:

l. A wave guide aerial, for the radiation or reception ofelectromagnetic waves having a compcnent of electric field along. theaxis of said guide, said guide having portions filled with dif ferentdielectrics, said wave guide'having an aperture on one side thereof:whereby the interior of said guide is coupled to the exterior thereof, adiscontinuity in the dielectric within said guide at the position ofsaid aperture, a portion of said exterior serving as radiating means,said wave guide being of reduced transverse dimensions in the portionwhich serves as a radiating means, the transverse dimensions of saidguide slowly decreasing from the section-of larger size to the sectionof reduced size, a plane boundary between said different dielectricsbeing disposed within the portion ofsaid guide which is or slowlydecreasing size, transverse dimensions of the guide at the boundarybetween said difierent dielectrics being so related: to the dielectricconstantsof the difierent dielectrics and the fre quency of the wavesthat reflection of said waves at said boundary is substantiall reduced.

2. A wave guide aerialfor the radiation or re ception ofelectromagneticwaves having a component of electric field along the axis of said guide,said guide having portions filled with different dielectrics, saidwaveguide having an aperture on: one side thereof wherebythe interior ofsaid guide is coupled to the. exterior thereof, a discontinuity in thedielectric withinsaid guide at the position of said aperture, aportionof said exterior serving asradiating means, said wave guide being ofreduced transverse dimensions in the portion which serves as a radiatingmeans, the transverse dimensions of said guide slowly decreasing fromthe section of larger size to the section of reduced size, a planeboundary between said different dielectrics being disposed within theportion of-said guide which is of slowly decreasing size and meansateach end of said radi-- ating portion for inhibiting the flow ofcurrent along the exterior of saidguide-beyond said radiating portion,transverse dimensions of the guide at the-boundary between saiddifferent dielectrics being so related to the dielectric constants ofthe different dielectrics andthe frequency of the waves that reflectionofsaid waves at said boundary is substantially reduced.

3. A wave guide aerial for the radiation or reception of electromagneticwaves having a component of electric field. along the axis of saidguide. said guide: having portions filled with different dielectrics,said wave guide having an aperture on one side-thereof whereby theinterior of said guide is coupled-to the-exterior thereof,

- a discontinuity:inthe dielectricwithinsaid guide at the position ofsaid aperture. a portion of said exterior serving as radiating means,said wave guide being of reduced transverse dimensions in the portionwhich serves as a radiating means, the transverse dimensions of saidguide slowly decreasing from the section of larger size to the sectionof reduced size, a plane boundary between said difierent dielectricsbeing disposed within the portion of said guide which is of slowlydecreasing size and means at each end of said radiating portion forinhibiting the flow of current along the exterior of said guide beyondsaid radiating portion, each of said means including a pair of conductivfins on said guide spaced apart a distance equal to a quarter of theoperating wavelength and extending away from the surface of said guide adistance equal to a quarter of the operating wavelength, transversedimensions of the guide at the boundary between said differentdielectrics being so related to the dielectric constants of thedifferent dielectrics and the frequency of the waves that reflection ofsaid waves at said boundary is substantially reduced.

4. A wave guide in the form of a hollow conductive tube for thetransmission of electromagnetic waves of the transverse magnetic typeand comprising two sections having different transverse dimensions, anintermediate section joining said first two sections, the transversedimensions of the latter section slowly decreasing from the section oflarger size to the section of smaller size, and different dielectricsdisposed within said first two sections and extending in to saidintermediate section with a plane boundary between said differentdielectrics disposed within said intermediate section.

5. The wave guide claimed in claim 4 having a circular internal crosssection.

EDWARD CECIL CORK. MICHAEL BOWMAN-MANIFOLD.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name 7 Date 2,129,712 Southworth Sept. 13,1938 2,142,138 Llewellyn Jan. 3, 1939 2,147,717 Schelkunofl Feb. 21,1939 2,197,123 King Apr. 16, 1940 2,202,380 Hollmann May 28, 19402,304,540 Cassen Dec. 8, 1942 2,407,690 Southworth Sept. 17, 1946

